Ha,ha — fire Mercury, “don’t kill my vibe, man”. My parents were both Mercury in Sagittarius and their fights could be epic, flame throwing all over the place, but they also enjoyed the humor of Mercury in Sag together and enjoyed traveling and reading together. I agree with your view on air Mercury with air Mercury. They do need words.
I have noticed that many of my best female friends who tell me the plain truth when I am seeking advice or an impartial opinion are Mercury in Virgo, but my Pluto conjunct North Node in Virgo may be why I am drawn to Mercury in Virgo people.
In general, in relationship, I look at the elements of the sign Mercury is in and same element is best, water/earth, or air/fire are “easy” because water feeds earth and air feeds fire. If Mercury/Mercury is air/water, air/earth, water/fire, or earth/fire, things can be more challenging, and I look at any other support or tension through other planets exchanging energy with both the natal Mercurys and how they interact in the synastry chart. House positions, as well, if they can create an affinity that might not be there in the elements. The water/fire is the mix that I see work the least. Water puts out fire and a fire sign Mercury can feel like a water Mercury is squelching their ideas.
When I see a natal Mercury in houses 4, 8, or 12, no matter what sign it is in, the water/intuitive/feeling/telepathic communication you mention regarding a water sign Mercury comes through so strongly.
The most intriguing Mercury to me is a fire Mercury in the 12th house. I see them as a “sleeper” Mercury. I think they are often underestimated or ignored, and they can self-describe as shy despite the fire, but when they find something they are passionate about or they decide to fight for a cause— look out!